This invention relates to colloidal dispersions of an alkaline earth metal salt in an oil-soluble acrylic polymer, which dispersions serve to improve the properties of lubricating oil compositions. This invention also relates to finished lubricating oil compositions having the metal salt-polymer dispersions of the invention incorporated therein, either alone or in combination with other oil additives, whereby the metal salt-polymer colloidal dispersions impart improved properties to the finished lubricating oil compositions.
Lubricating oil compositions for internal combustion engines and for automatic transmissions require a combination of additives in the oil base to supply various functions, in addition to lubricity, such as rust and corrosion inhibition, oxidative and thermal stability, dispersion of carbonaceous deposits and insoluble matter formed by fuel combustion and oil oxidation, neutralization of acidic materials formed by oil oxidation, and improved viscosity index.
It is known in the art to employ basic metal compounds dispersed in an oil additive composition in a lubricating oil composition to provide an alkaline reserve in the lubricating oil composition so as to neutralize acidic materials formed therein. The preparation of such dispersions is commonly referred to in the art as "overbasing," which term was originally derived as a description of the practice of incorporating an alkaline earth metal compound in an oil base in excess so as to satisfy the equivalency of any acidic materials produced by oxidation of oil and burning of petroleum distillate fuels. Examples of such known basic metal compounds are alkaline earth metal salts of organic sulfonic acids, alkyl phenols (phenates), alkyl thiophenols, sulfides, and carboxylic acids.
Although the metal sulfonates and phenates satisfactorily serve to neutralize acidic oil by-products and contribute to detergency and corrosion inhibition, they can themselves contribute to undesirable insoluble deposits once their function is exhausted. The use of carboxylic acid salts such as, for example, alkenyl substituted succinates, and of high molecular weight carboxylic acid esters in combination with metal salts of acidic gases such as, for example, CO.sub.2, has provided some improvement in the art in that such compounds decompose upon exhaustion to give substantial amounts of volatile by-products, thereby leaving reduced amounts of insoluble deposits. The use of nitrogen-containing derivatives of high molecular weight carboxylic acids along with metal salts of acidic gases such as CO.sub.2 has provided further improvement in the art in that the presence of the nitrogen-containing moiety contributes dispersancy to additives possessing detergency, rust and inhibiting, oxidative and thermal stabilizing, and acid-neutralizing properties. For example, overbased alkenyl succinic anhydride derivatives, including the class consisting of the reaction products of the anhydride with amino compounds and nitrogen-containing hydroxy compounds, are commonly used. Still further improvement in the art has been achieved by including in lubricant mixtures compounds which are characterized by excellent resistance to viscosity breakdown under shearing stress, i.e. viscosity index improver additives. High polymer compounds such as, for example, polyalkylmethacrylates, are a class of compounds which is particularly useful in lubricant formulations in that these compounds demonstrate especial viscosity index improver effects in addition to dispersancy, detergency, oxidative stabilization and compatibility with additives in lubricating oil compositions.